There are optical communication techniques in which a quadrature phase shift keying (QPSK) signal, a dual polarization quadrature phase shift keying (DP-QPSK) signal, a quadrature amplitude modulation (QAM) signal, and the like are used. In these modulation methods, when a received signal is obtained, such as phase-modulated signal or quadrature amplitude-modulated signal and local oscillator (coherent optical wave) that serves as a reference are made incident on an optical 90-degree hybrid circuit, the signal and the local oscillator are being interfered, and interfering signal converted to an orthogonal relation is output as illustrated in FIG. 1. The interfering signal is detected by the optical receivers, and an in-phase (I) signal, a quadrature-phase (Q) signal, and the like are output.
In the optical 90-degree hybrid circuit, as illustrated in FIG. 2, polarized multiplex signal with a rotation state of random polarization is input to the 90-degree hybrid circuit, and the polarized multiplex signal is split into a vertical polarization component and a horizontal polarization component by a polarization splitting element. The polarization-split vertical polarization component and horizontal polarization component are converted to a 45-degree sloped polarized wave and a 135-degree sloped polarized wave respectively by using a polarization conversion element (λ/2 wavelength plate) to rotate the planes of polarization 45 degrees.
Meanwhile, the local oscillator is converted to a circularly-polarized wave by using a polarization conversion element (λ/4 wavelength plate), and is multiplexed with two signals optical signals that have been converted to planes of polarization having slopes of 45 degrees and 135 degrees by using an optical multiplexing and de-multiplexing device (half mirror).
The multiplexed optical signal is then de-multiplexed, and two optical signals that are put into a differential relation (in phase and out of phase) by an individual optical multiplexing and de-multiplexing device are output. Four output optical signals in total output from two optical multiplexing and de-multiplexing devices are respectively input to the polarization splitting element, and thus are split into polarization-split IQ (In-phase, Quadrature-phase) signals. An example of constituting the above-described optical 90-degree hybrid circuit using a space optical system is described in patent literature No. 1.
In an optical receiver, the rating of an input signal to a section that amplifies an electric signal (Trans Impedance Amplifier; TIA) is set, and therefore it is not possible to receive an optical signal with a certain amount of power or more. For optical signal being input to the optical 90-degree hybrid circuit, the power ratio between the local oscillator and the signal needs to be sufficiently increased in terms of the signal qualities, which is limited by the input rating of the TIA.
The electric signal input to TIA (the value of an optical current output from the optical signal receiving element) is expressed as follows:Electrical signal input to TIA ∝(signal power×local oscillator optical power)^0.5  (Expression)
Since the rating of TIA is within a certain range, in a case in which the power of the signal is increased, it is necessary to decrease the power of the local oscillator. To maintain the signal qualities after demodulation, it is necessary to sufficiently increase the power of the local oscillator compared with the power of the signal being input while the input rating of TIA is maintained. For example, in a case in which the signal power is excessively great, a sufficient power ratio between the local oscillator and the signal cannot be obtained.
In a case in which the power of the signal is adjusted while the power of the local oscillator is maintained to be constant, specifically, it is possible to adjust the power of the signal by using a variable optical attenuator (VOA) provided ahead of the optical 90-degree hybrid circuit as illustrated in FIG. 3. However, since the number of devices in a receiving system is increased due to the variable optical attenuator, there is a problem in that the constitution becomes large and complex.
In addition, in a case in which the variable optical attenuator is not used as illustrated in FIG. 1, the optical power ratio of the local oscillator to the signal is maintained at a certain level or higher, and therefore it is necessary to use a local oscillator source equipped with an optical power adjustment function. In this case, it becomes impossible to receive a signal having a certain amount of power or more due to the limitation by the above-described input rating of TIA.